Add option to run `pg_dump` into `migrations/schema.sql` on commit by default

### Feature description

The README currently recommends doing this yourself, and provides a straightforward [example](https://github.com/graphile/starter/blob/4854f77e461062a95cdfff9c62082eb90a3a0d5b/%40app/db/.gmrc#L20), but IMO this should be built-in and ideally enabled by default once proven out.

My thinking is that reviewing the final `schema.sql` should be core to the workflow of using `graphile-migrate` – this is an opinionated tool, and it should be okay to enforce this opinion (especially because some of its other opinions don't work so well without it, like the fact that you can accidentally leave a `rnd()` function lying around). I don't think a tool like this should be used _without_ dumping and reviewing the schema.sql on every commit.

Ideally, any issues people have with this workflow should also be reported and fixes shared, rather than worked around in each user's bash scripts.

Disclaimer that I am a prospective user, not an active user – this is more a suggestion than a request.

### Motivating example

### Breaking changes

### Supporting development

I [tick all that apply]:

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [ ] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [x] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️ (patreon)
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile